The main objectives in the production of such papers are their qualitative properties in terms of strength, impregnation behavior, varnishability and printability, which are necessary for the further processing steps, but also the optical goals of achieving the required and specified coloring. In all cases, the paper has to be provided with color thoroughly and in depth. Coating base papers are produced in all degrees of color/saturation/brightness that might be obtained metrologically from the entire color spectrum.
Coating base papers, sometimes also referred to as decor base papers, are highly technical special papers which are printed on with aqueous or solvent containing dye systems or which are processed further in an unprinted or monochrome form. This applies to all conventional printing processes such as gravure printing, offset printing, flexographic printing, screen printing, but also to all non-impact printing processes such as digital printing systems. The further processing may be divided essentially into the processes of impregnating, painting, pressing onto wood-based materials or lamination onto wood-based materials or other sheetlike materials.
Wood-based materials are chipboards, fiberboards, medium density fiberboards (MDF) and high-density fiberboards. However, it is also possible to coat or laminate boards made of a whole variety of other materials such as, in particular, mineral materials, plastics or metals.
Another type of further processing of such papers is the production of decorative laminate boards, which are produced from impregnated, printed and/or deeply through-colored coating base papers and core papers by being pressed to a homogeneous board, or which are produced in an endless process [1].
Coating base papers have to be producible in all the colors of the color spectrum that can be perceived by the human eye, including the highest brightness (white) and the highest darkness level (black). In order to achieve a specific color at a specified color location along with certain physical properties, organic and inorganic pigments of various particle sizes are used with different mixing ratios and concentrations. To meet and maintain all of the physical conditions and requirements, fillers are used additionally.
An important pigment that is used to improve the brightness and opacity of the paper is titanium dioxide (TiO2). In general, titanium dioxide is added to the fibrous paper in a “wet-end process” (see for example WO 2013/109441 A1).
Coating base paper provided as a fibrous substrate is the most economical, flexible and functional solution for providing designed and styled surfaces for a wide variety of applications such as furniture for living and sleeping areas, kitchens, offices, bathrooms, floors, interiors of large objects such as airports, hotels, office buildings, buildings of public interest such as museums, galleries (see for example WO 2013/109441 A1).
Coating base paper needs to have a very high opacity which should be as close as possible to 100%. The coating capacity against the background, i.e. against the color of the substrate material, shall be ensured without loss of color impression. Crucial factors to reach this goal are the content (amount) and the distribution of pigments and fillers within the paper body. The limiting amount is predetermined by the requirements regarding the strength of the paper.
It is basically known that the limiting amount can be raised by increasing the areal density of the paper. Thus, if the areal density of the paper is high enough, the desired 100% opacity can almost be reached. According to the known state of the art, there are commercial limits for the reasonable use of pigments and fillers.
The most commonly used pigments, i.e. white (titanium dioxide) and colored (iron oxides), represent a high value and are subject to immense, cyclical price fluctuations. Therefore, reaching a maximum yield is very important. This in turn means that the pigments/fillers in the paper body must have a maximal particle distribution in order to achieve the best possible opacity and the best coating capacity. Up to present it has not been possible to reach this standard. The pigments/fillers are generally present in the paper body as agglomerates. As a consequence, the light-scattering layers overlap and reduce the opacity effects and give rise to a different color perception.
In order to reduce the agglomeration phenomena, specific binders, fillers or dispersants are used, whereby an improvement of the light scattering efficiency is achieved [2]. However, in view of the increasing importance of environmental concerns and also because of the increasing costs of the raw material, new solutions are being worked out which should lead to a reduction of the titanium dioxide requirements through the use of biomaterials.
Accordingly, it is an object of the present invention to provide a fibrous substrate material, in particular a coating base paper, which stands out for high quality, in particular for high opacity, low requirement for pigments and good mechanical stability. A further object of the present invention is to provide a method for producing the substrate material according to the present invention. As a further object of the present invention, there is provided a coating base paper or a prepreg with improved properties.